1. Field of the Invention
The present invention relates to novel compositions based on ceric oxide having a thermally stable specific surface and to the use of such cerium oxide-based compositions, notably in the field of catalysis.
By "specific surface" is intended the B.E.T. specific surface as determined by the Brunauer, Emmett and Teller technique described in Journal of the American Chemical Society, 60, 309 (1938).
2. Description of the Prior Art
It is known to this art that ceric oxide is a useful catalyst or catalyst support. Compare, for example, the text by Paul Meriaudeau et al, relating to the synthesis of methanol from CO+H.sub.2 in the presence of catalysts of platinum deposited onto ceric oxide. (C. R. Acad. Sc. Paris, v.297, Series II-471 (1983)).
It is also well known to this art that the efficiency of a catalyst is generally proportionately higher when the area of contact between the catalyst and the reactants is large. To accomplish this, the catalyst must be maintained in a state which is as finely divided as possible, namely, the solid particles comprising same must be as small and individualized as is possible. The fundamental role of the support, therefore, is to maintain the catalyst particles or crystallites in contact with the reactants in as finely divided state as can be attained.
Over the extended life of a catalyst support, its specific surface decreases, due, on the one hand, to the coalescence of the very fine micropores thereof and, on the other, to the enlargement of the divided particles, for example by sintering. During this decrease in surface area, a fraction of the catalyst is absorbed into the bulk of the support and is no longer available for contact with the reactants.
To date, most of the available ceric oxides have a specific surface which decreases rapidly in the event of operating temperatures higher than 500.degree. C. Thus, R. Alvero et al, J. Chem. Soc. Dalton Trans., 87 (1984), describe the preparation, from ceric ammonium nitrate, of a ceric oxide exhibiting a specific surface of 29 m.sup.2 /g after calcination at a temperature of 600.degree. C.
In the aforesaid P. Meriaudeau et al publication, a CeO.sub.2 is prepared having a specific surface of 27 m.sup.2 /g via essentially the same process.